1. Field of the Invention
This invention relates to an apparatus for the extrusion of oleaginous plant materials, or oilseeds, as a preparatory step to solvent extraction of oil from the oilseeds. In particular, this invention relates to an extruder having means for draining oil liberated from the plant material during the extrusion of the material.
A standard method of obtaining oil from oleaginous materials such as oilseeds in solvent extraction. Extrusion is sometimes used as a preparatory step to improve the properties of the material which is treated in large-scale commercial solvent extraction systems. For example, oleaginous plant materials like rice bran, which are troublesome in solvent extraction because of their fine particulate nature which retards the flow of solvent through the material thereby reducing the efficiency of the extractor, are converted by extrusion into porous collets which allow for much faster flow of solvent through the material. Other oleaginous plant materials, such as soybean, are often flaked prior to solvent extraction, but the flakes have a low bulk density and tend to fall apart during solvent extraction, preventing an adequate drainage of solvent from the solids residue (marc) leaving the extractor. Extrusion converts the flakes into porous collets having greater bulk density than flakes, which allows for an increase of capacity flowing through the extractor without changing the bed depth or extraction time within the extractor. The collets have greater strength than flakes and do not fall apart so easily, which allows the marc to drain better before it exits the extractor.
Some oleaginous plant materials contain high levels of oil, or fat, as, for example, peanuts, safflower, rapeseed or canola, and copra. These materials are typically crushed in screw presses as a first step, to help rupture the cells containing the oil and to remove from the material a significant portion of the oil. The partially de-oiled residue is then cracked, or flaked, and sent directly to a solvent extractor, or it is processed through an extruder first before going to the extractor to attain larger, firmer, collets and/or to attain higher bulk densities.
Extrusion has been very effective in improving the solvent extractability of many oleaginous plant materials and is well established in the preparation of soybean, rice bran, cottonseed, and prepressed canola, sunflower and other oilseeds. There are, however, some problems in the extrusion of some oleaginous plant materials.
One problem with present extruders is that, if the oil or fat level of the material going into the extruder is above about 30% by weight, some of the oil is liberated within the extruder. This interrupts the steady-state operation of the extruder by creating pockets of free oil randomly spaced within the matrix of solid residue. The pockets of free oil exit the extruder at high velocity and interrupt the flow of collets. This also causes an undesirable loss of oil, the oil being the principal product sought during solvent extraction.
Another problem with extruders currently used in the oilseed industry is related to the low bulk density of the flaked material entering the extruder. When extruders are used to process materials besides oilseeds, for example, pet foods, the material being fed into the extruder is granular and at a relatively high bulk density, around forty pounds per cubic foot. For the treatment of oilseeds, on the other hand, the feed is usually flaked, and is therefore at a lower bulk density, around twenty-five pounds per cubic foot, because of the air voids between the flakes. Thus, because of the shape of the flakes, a great deal of air is drawn into the extruder along with the solids. This is a handicap because the feed worm thus cannot feed enough solids to the compaction worms in order to utilize the full capacity of the extruder and the total applied horsepower. This combination of low bulk density and the presence of air causes oilseed extruders to operate at a lower overall capacity than they otherwise could were the solids throughput or efficiency of the feed worm increased.
2. Description of the Prior Art
For a long time prior to and after World War II, the traditional methods of recovering oil from oil bearing materials, both vegetable and animal materials, were (i) screw pressing to residual oil levels of approximately 3% to 10% by weight of the pressed residue, or (ii) direct extraction in solvent extractors to a residual oil level below 1%.
A typical screw press is described in U.S. Pat. No. 2,249,736. The screw press is an apparatus having a rotating shaft within a cylindrical barrel having slotted walls. The shaft exerts pressure upon the oleaginous plant or animal tissue material by trying to force it through a restricted opening at the discharge end of the barrel. The pressure releases the oil from the cells contained in the tissue by rupturing the cells. The liberated oil flows out through the slotted walls of the barrel.
A typical solvent extraction apparatus is described in U.S. Pat. No. 3,159,457. The material to be treated is transported into moving baskets which pass under piping which sprays solvent into the baskets. This causes the oil to be dissolved and leached out of the oil-bearing material. This type of extractor, the percolation basket extractor, is the type most commonly used in the extraction of oil from oleaginous plant and animal materials.
However, many oilseeds cannot be directly extracted because the oil is bound too tightly within the plant tissue; because the plant tissue lacks strength when it is reduced in size to form thin flakes suitable for extraction; or because its oil level is high enough to interfere with the formation of flakes. A procedure was therefore developed involving the combination of screw pressing and solvent extraction, wherein the high-oil-containing, hard-to-extract material, was first passed through a screw press and subjected to mild pressing in order to lower the residual oil to a level equal to about one-fourth of the total protein content of the material. The action of the press helped to liberate the oil from the plant tissues and helped to convert some of the constituents within the plant material into a gel-like state which imparted greater strength to the material when it was formed into flakes subsequent to the screw pressing. Such a method is described in U.S. Pat. No. 2,551,254. This method allowed for some oilseeds, that were previously full-pressed to low residual oil levels using high compression screw presses, to be processed at higher volumes, in a less labor-intensive procedure.
By the early 1960's the labor and maintenance requirements of screw presses rose high enough to stimulate an interest in procedures that would allow for the elimination of screw pressing altogether For example, one major oilseed that had been pre-pressed and solvent extracted (cottonseed), was now flaked and sent directly to solvent extraction. Direct extraction required a longer extraction time, and didn't result in as low a residual oil level as pre-press solvent extraction did, but it was considered a step forward because it phased out the labor-intensive screw presses
Oleaginous plant materials have since then for a number of years been formed into porous collets prior to solvent extraction, by means of extrusion using extruders that have closed barrels. An example of an extruder used for this application is described in U.S. Pat. No. 3,108,530. An example of the procedure for forming the collets, and inactivating enzymes, etc., is described in U.S. Pat. No. 3,255,220.
In an extruder, the solid matter of the material passes through the extruder and is subjected to increasing pressure and temperature as it is worked towards the discharge end, and, by the time the material reaches the discharge end, it is compacted into a compressed mass. The entire mass of material flows through at least one orifice on the discharge die plate of the extruder into normal atmospheric pressure.
When the material flows out of the extruder into atmospheric conditions, it may expand because of the vaporization of moisture contained in the tissue. There is some swelling of the material due to the sudden drop in pressure as it leaves the extruder, but "expansion", as the term is used herein, is caused by the production of minute pores and cavities by the vaporization of moisture contained within the tissue of the oilseed material. These pores and cavities cause the material to become permeable. The material is thus made quite suitable for solvent extraction.
Inside the extruder, the material is heated to a point where the vapor pressure of the water content of the material is significantly in excess of atmospheric pressure; the water, however, is held in the liquid phase by the pressure of compaction within the extruder. When the material exits the extruder into atmospheric conditions, some of the water instantly vaporizes. This occurs wherever the water is, and the water is distributed evenly throughout the material.
The amount of water that vaporizes is dependent upon the temperature of the material. It takes approximately 970 BTUs to vaporize one pound of water at atmospheric conditions. The BTUs come from the heat of the extruded material. For the liberation of 1 BTU, the temperature of one pound of water, or approximately two pounds of fat, or approximately four pounds of solids, must be lowered one degree Fahrenheit. One can calculate how many BTUs are available for vaporization by multiplying the drop in temperature (from extrusion temperature to atmospheric temperature) by 1 BTU for each pound of water, 1/2 BTU for each pound of fat, and 1/4 BTU for each pound of solids contained in the material being extruded. The amount of BTU's per hour that are available is then divided by 970 to come up with the pounds of water that will vaporize per hour. It is this vaporization of water that is the driving force causing the "expansion" of the material.
The heat input into the material comes from the injected steam and from friction generated by the shaft. The heat from steam is blended into the material a short distance downstream of the steam valves, but the heat generated by friction arises all along the surface of the shaft, with the major portion of it occurring near the downstream end of the shaft where the compaction is greatest. In order to monitor operating conditions, there is usually a thermometer, such as a dial thermometer, placed in the breaker screw position preceding the last compaction worm. Although this is a convenient place to locate a thermometer, it does not detect the highest temperature attained in the extruder. The highest temperature is attained after the last compaction worm, with some additional frictional heat generated as the material is forced to flow against the drag of the dies.
Therefore, it is possible to "expand" a product, yet register a temperature at the thermometer lower than the boiling temperature of water under atmospheric conditions. Applicant has observed an expander in operation on soybean making an acceptably "expanded" product with a temperature of 190.degree. F. (87.7.degree. C.) registering on the dial thermometer. It should be understood that, regardless of the temperature displayed on the thermometer, "expansion" to produce a porous interior cannot occur unless the vapor pressure of the contained water, or other volatile constituent, significantly exceeds the vapor pressure of that constituent under the atmospheric conditions prevailing when the material exits through the dies.
In the mid 1960's, the use of an extruder to prepare oleaginous plant materials, as mentioned above, was first applied on rice bran for the agglomeration of the finely divided rice bran fragments into porous collets and for the inactivation of the enzyme lipase, which caused a rapid deterioration of the rice oil.
During the 1970's, extrusion began to be applied to soybeans. Soybeans up to that time had been flaked and sent directly to solvent extraction. There were no particular technical problems with the direct extraction of soybean; it was considered an easy material to directly extract because it was fairly low in oil (18%) and was easily rolled into thin, durable flakes. But some processing plants were looking for means to increase their soy oil production beyond the capacity rating of the extraction equipment. Extrusion of the soybean flakes, to convert the flakes into collets having greater bulk density than the flakes and less tendency than flakes to fall apart into fine particles, allowed a plant to achieve a 50% to 100% increase in capacity. The use of extrusion thus spread rapidly in the soybean crushing industry during the 1970's.
Soybean and rice bran both contain less than 20% by weight of oil, and present no problems with the liberation of free pockets of oil during extrusion. Soybean, with about 18% oil, would have some of the oil liberated inside the extruder, causing the extrudate exiting the extruder to be sometimes covered with a frothy coating of oil containing a foam of boiling water as some of the moisture escaped from the solid matrix. After an initial flashing of moisture, the extrudate would cool and the boiling cease, and the oil would then be reabsorbed into the solid material.
Such extruders, as described above, all find application on oilseed materials containing less than approximately 30% oil by weight. If an oilseed containing more than about 30% oil by weight is processed in such an extruder, however, there is a likelihood that some of the oil will be liberated within the extruder and not reabsorbed, forming pockets of free oil which squirt out of the dies and interrupt the steady-state operation of the extruder. If this problem is encountered to a minor degree, it may be corrected by adding some finished meal, from which the oil has already been extracted, into the inlet of the extruder to mix with the incoming material and dilute its oil level down to a point where all of the liberated oil will be reabsorbed into the solids. If this problem is encountered to a major degree, the oil level must first be reduced by pressing the material in a screw press before sending it through the extruder
Rapeseed (containing about 42% oil by weight) and other oilseeds, with oil levels higher than above 30%, therefore do not readily lend themselves to extrusion because of this problem, but must be screw pressed first to around a 15-25% oil level and then extruded. However, there is a strong interest in the oilseed crushing industry to phase out screw presses completely because they are perceived as high-wear, labor-intensive, and low-capacity devices.
Thus, it has become desirable to find a way to process material having a high oil content, in an extruder, without having to put it through a separate screw press first.
U.S. Pat. No. 4,361,081 describes an extruder for processing oilseed and having a perforated barrel wall section for drainage of oil therefrom. This patent, however, does not make any reference to extrusion of material at a high enough pressure to keep any water in a liquid phase until it exits the die plate. Thus, this apparatus does not provide for the expansion (of the compressed material) and porosity caused by vaporization of moisture content, which are so desirable for later solvent extraction.
A screw press modified to include an extrusion chamber at the discharge end has recently been introduced to the oilseed crushing industry. It is described in U.S. Pat. No. 4,646,631. It is substantially a screw press, very similar to the screw presses already in use for pre-pressing oilseed materials, but having a closed wall section at the end of the press with a die plate for product discharge rather than the annular choke mechanism most screw presses employ. The oilseed material is processed through the screw press section in much the same way it would be through a stand-alone screw press, pressing at the same moisture-temperature conditions and to the same residual oil level. Then, when the material continues downstream past the screw press section and enters the extruder section, moisture is injected to elevate the moisture level closer to that commonly used in extrusion; and the moistened material is extruded through die openings similar to those used in conventional extruders, with vaporization of any water which has been kept in the liquid phase because of high pressures in spite of temperatures over 100.degree. C. The idea is to try to combine both devices, a screw press and an expander/extruder, onto a single shaft so that one machine can take the place of both.
There are a number of inherent difficulties with a device as described in U.S. Pat. No. 4,646,631, however. First, the device is still primarily a screw press and still has the inherent shortcomings of a screw press, namely that it is a high-wear, labor-intensive, and low-capacity device. Moreover, it is difficult to select a compromise rotational speed for the common shaft. Stand-alone expander/extruder shafts generally rotate 4 to 6 times faster than stand-alone screw press shafts. For example, typical expander/extruder shafts rotate at 220 to 440 RPM, whereas screw press shafts rotate from 35 to 100 RPM.
It is also difficult to match the horsepower expended into the product by the two machines Screw pressing to 15-25% oil typically consumes 0.9 to 2.0 HpD/ton. (Horsepower-Days/ton can be illustrated by the following: A capacity of 100 tons per day [of material entering the screw press] would require the consumption of 90 to 200 hp. A known press is rated for 170-200 T/D cottonseed or sunflower seed, which would pass 125-160 T/D of meats into the screw press and which requires a 225 Hp motor. 225 Hp/160 T/D=1.4 HpD/ton power consumption.) Extrusion, on the other hand, does not consume as much horsepower. Its power consumption is typically 0.2 to 0.5 HpD/ton. A 225 Hp expander/extruder could, therefore, have a capacity of 450 to 1,125 tons/day, much greater than that of an equivalently powered screw press.
An extruder consumes less horsepower than a screw press because the oilseed material is at a higher moisture level all the way through the extruder. This elevated moisture level makes the oilseed less abrasive, and this factor, coupled with the reduced horsepower consumption, makes an extruder less subject to wear than a screw press, and less subject to maintenance because of wear. And, because of the faster rotational speed, and lower horsepower requirement, a relatively inexpensive machine can have a considerably higher throughput than a screw press of the same cost.
A screw press also requires more operator attention than an extruder. A screw press generally is equipped with an adjustable choking mechanism located at the discharge end of the barrel serving as a means to enlarge or reduce an annular opening through which the solid residue exits the press. When the choke is opened, pressure is reduced. When it is closed, pressure is increased, more oil is pressed out, and the solid residue is harder and more compacted. The choke is opened to facilitate start-up and shutdown, and is adjusted during operation to cause enough pressure to bring the residual oil level into an acceptable range. If the residual oil level drifts, because of a drift in the moisture, temperature, or purity of the material entering the press, the choke is adjusted to compensate for it. Also, when pressing to a 15 to 25% residual oil level, there is sufficient pressure exerted within the screw press to cause some of the solids to flow out with the oil. These tend to accumulate on the exterior of the barrel drainage areas, and have to be scraped off manually by the operator.
Extrusion, on the other hand, requires less operator attention. Fixed dies are used rather than an adjustable choke, because an extruder is less sensitive to drift than a screw press. Steam is injected into an extruder to adjust for optimum product. If drift occurs, the steam flow can be readjusted, the concern being to add enough steam to prevent the main drive motor from overloading. Since motor amps are easily measured on stream, whereas residual oil cannot be measured on stream, it is easy to provide an automatic controller which will automatically adjust steam flow to prevent main drive motor overload.
Accordingly, it would be most desirable to be able to utilize an extruder to directly pretreat high-oil-content materials, yet at a sufficiently high throughput rate to more completely utilize the capacity of the extruder.